Capacitive pressure transducer and method of making same

ABSTRACT

A capacitive pressure transducer comprising a pair of disc shaped members having planar surfaces made from an insulator material, one of the members being several times thinner than the other plate and flexible and constituting a diaphragm and the other thicker plate constituting a stationary plate. A thin conductive film is formed on the surface of each of the members to form the plates of the capacitor. A glass frit is applied on the marginal edge of each member and when the members are held in adjacent relationship, the assembly is fired to seal the two members together while spacing them a predetermined distance apart so that the two conductive plates are opposite each other and are separated by an open gap of a predetermined distance, the two conductive plates being insulated one from the other. Leads are electrically connected to the conductive plate of each disc through the fused glass frit. When pressure is applied to the member, the diaphragm member is displaced thereby changing the capacitance of the pressure transducer.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 898,469, filed Apr. 20, 1978, now abandoned which is acontinuation-in-part application of U.S. application Ser. No. 834,498filed Sept. 9, 1977 now U.S. Pat. 4,177,496 which is a continuationapplication of U.S. application Ser. No. 666,188 filed Mar. 12, 1976 andnow abandoned.

FIELD OF INVENTION

This invention relates to pressure transducers and more particularly tocapacitive type pressure transducers utilized in conjunction withelectronic circuits.

DESCRIPTION OF THE PRIOR ART

It is frequently necessary to determine the pressure of gases andliquids for purposes of measurement or control. Furthermore, it is alsofrequently necessary to provide means for sensing the pressure of fluidsand gases in engines, machinery and equipment. Accordingly, there existsin the prior art many devices for measuring pressure. One type of suchdevice uses the piezoelectric principle. When pressure is applied to thepiezoelectric device, a voltage which is proportional to the pressureapplied is generated by the device. While the piezoelectric device doesallow one to measure pressure, it is best used only for measuringchanges in pressure and not static pressure. Furthermore, piezoelectricdevices are relatively insensitive and have a low accuracy.

Another device for measuring pressure is the strain gauge. Strain gaugesare resistive devices in which resistance changes in a manner related tothe pressure applied thereto. Strain gauge devices can measure staticpressure but there is a small percentage change for large pressurechanges. Furthermore, strain gauges lack stability with temperature andtime.

Another type of device utilizes resilient resistive materials whichchange resistance in proportion to the pressure applied thereto. Suchdevices also measure static pressure and are more sensitive than thepiezoelectric devices but also lack stability with temperature and time.

There are also capacitive devices whose capacitance varies with changesin pressure. These capacitive devices have better stability withtemperature and time than those devices previously discussed and aremore sensitive than strain gauges, but require a very complexmanufacturing procedure and as such are very expensive. Furthermore,since such devices typically utilize different materials, there areproblems created by the different rates of thermal expansion resultingin a lack of measurement uniformity.

Still another type of pressure sensor in use today utilizes a capacitivetype transducer which includes a diaphragm on which a conductive plateis mounted, the diaphragm being arranged in operative association withanother conductive plate which may be deflectable or stationary. Thesetwo plates are suitably disposed in spaced-apart relationship to formthe plates and thus the capacitance of the transducer as a function ofpressure. The capacitor plates are coupled into a suitable electricalnetwork so that the changes in capacitance are transformed into outputsignals which reflect the magnitude of the pressure measured by thetransducer.

One conventional type of such capacitive-type pressure transducerutilizes a pair of complementary members which are sealed together toform a unitary structure. The members are shaped to define a hollowinterior and the adjacent surfaces of the members within the interiorare provided with a planar conductive material to form the two plates ofa capacitor. One of the members generally comprises the flexiblediaphragm with the other member being formed of rigid material. Such acapacitive-type pressure transducer is shown in U.S. Pat. No. 3,748,571wherein a diaphragm having a surface coated with a layer of conductivematerial, is provided. The diaphragm is supported by a cup member bondedin sealed relationship to another cup member so that the conductivelayer in the diaphragm is disposed in spaced relationship with a layerof conductive material formed on the surface of the other cup member.The conductive layers form the plates of a capacitor, the spacing ofwhich is varied by the flexing of the diaphragm. The capacitor formed bythe conductive layers is connected by means of leads extending throughthe cup members for connection at one end to the conductive layers andto an associated electrical network from which output signals, asdetermined by the spacing of the capacitor plates are obtained.

A similar pressure sensor construction is shown in U.S. Pat. No.3,952,234 wherein spaced capacitor plates are provided on a diaphragmand stationary plate respectively to form a differential capacitor, thestationary plate being provided with a pair of plates to compensate forimbalance of the capacitor during deflection of the diaphragm. In U.S.Pat. No. 3,750,476, the pressure transducer utilizes capacitor platesformed by metallic coatings on a rod and a tube in which the rod isdisposed. A suitable annular space is provided between the rod and tubeto form a capacitor gap. Flexing of the outer tube with pressure variesthe spacing between the sleevelike capacitor plates providing a signalcorresponding to the pressure sensed. All of the aforementionedcapacitive-type pressure transducers utilize discrete conductors whichare conducted through suitably provided passages in the housing orsupport structure in which the capacitor is housed and must be connectedto the plates by a process such as soldering or the like. Therequirement of such passages and separate leads materially increased thecomplexity of the transducer and adds to the manufacturing cost.Furthermore, such designs decrease the sensitivity of the capacitorand/or its linearity over the operating range of the capacitor.

Applicants are also aware of the use of a glass frit to seal electronicdevices together. However, applicants are not aware of the use of aglass frit to seal and separate capacitor plates as envisioned in thepresent invention.

SUMMARY OF THE INVENTION

A capacitive pressure transducer constructed with a unique combinationof a pair of planar disc shaped plates made from an insulating material.An electrically conductive film is screened onto each of the plates toform the plates of a capacitor and conductive areas accessible for leadattachment, and glass frit is placed on the marginal portion of thediscs so that when the discs are placed together in an overlyingrelationship such that the two conductive films are opposite each other,the glass frit is fired to seal the discs together and space them apartso that they are separated by an open gap of predetermined distance andare insulated from each other. One of the discs being thin enough to beflexible, functions as a diaphragm, while the other thicker plate isrelatively rigid.

When pressure is applied to the diaphragm, the diaphragm is displacedthereby changing capacitance which can be detected by an electroniccircuit as an indication of the value of the pressure applied to thecapacitive pressure transducer.

Thus it can be seen that the present invention provides a capacitivepressure transducer which is reliable, easy to manufacture and low incost.

The construction of the invention results in a simplified method ofmaking a capacitive pressure transducer which is very sensitive tochanges in pressure with a high degree of accuracy and stability overlong periods of time and under wide temperature ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned description of the invention and other features andobjects of the present invention will become more apparent by referenceto the following description taken in conjunction with the accompanyingdrawings, wherein like reference numerals denote like elements and inwhich:

FIG. 1 is a top plan view of a capacitive pressure transducer inaccordance with the teachings of the present invention;

FIG. 2 is a cross-section of the embodiment of FIG. 1 taken along the2--2 line;

FIG. 3 is another embodiment of a capacitive pressure transducer inaccordance with the teachings of the present invention shown incross-section;

FIG. 4 is a third embodiment of a capacitive pressure transducer inaccordance with the teachings of the present invention shown incross-section;

FIG. 5 is a fourth embodiment of a capacitive pressure transducer inaccordance with the teachings of the present inventin shown incross-section;

FIG. 6 is a fifth embodiment of a capacitive pressure transducer inaccordance with the teachings of the present invention shown incross-section;

FIG. 7 is a sixth embodiment of a capacitive pressure transducer inaccordance with the teachings of the present invention shown incross-section;

FIG. 8 is the capacitive pressure transducer of FIG. 7 provided with anelectronic circuit.

FIG. 9 is a specific embodiment of a capacitive pressure transducerderived from FIGS. 3 and 8 in accordance with the teachings of thepresent invention shown in cross-section;

FIG. 10 is a preferred embodiment of a capacitive pressure transducer inaccordance with the teachings of the present invention shown incross-section;

FIG. 11 is a top plan view of the stationary disc of the capacitivepressure transducer shown in FIG. 10;

FIG. 12 is a top plan view of the diaphragm disc of the capacitivepressure transducer shown in FIG. 10;

FIG. 13 is an exploded perspective view of the pressure transducerassembly utilizing the pressure transducer shown in FIG. 10-12; and

FIG. 14 is a perspective view of the assembled pressure transducer ofFIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring more specifically to the drawings, FIGS. 1 and 2 are anembodiment of a capacitive pressure transducer in accordance with theprinciples of the present invention.

In FIGS. 1 and 2, the capacitive pressure transducer includes two platesin the form of thin planar surfaced discs 2 and 4 made from anonconductive or insulating material. Each of the discs 2 and 4 has athin central electrically conductive metalized layer 6 and 8. Thecentral electrically conductive layers 6 and 8 are substantiallycircular in shape with substantially identical parallel diameters. Otherconductive layers 10 and 12 are metalized respectively onto each of thediscs 2 and 4 and form a conductive path from the central layers 6 and 8to the outer edge of discs 2 and 4. Leads 14 and 16 are coupled andelectrically connected to conductive layers 10 and 12 respectively.

The capacitive pressure transducer 21 is assembled by bonding one sideof both discs 2 and 4 to a spacer member 18 made from a nonconductive orinsulating material so that the central conductive layers 6 and 8 arepositioned opposite each other and separated by a gap 7.

In practice discs 2 and 4 and spacer member 18 may be made from anynonconductive material but the material selected for each part should bethe same material or a substantially similar material. In thisembodiment the nonconductive or insulating material is preferably onewhich has approximately a zero hysteresis, such as alumina, fusedsilica, or glass such as Pyrex. Furthermore, the conductive layers maybe plated on, etched on, sputtered on, screened on and fired, or appliedin any other manner well known in the art. Since in some applications anabsolute pressure measurement is required, the gap 7 between conductivelayers 6 and 8 which form the plates of the capacitor may be evacuated.Also discs 2 and 4 can be bonded to spacer member 18 by applying a smallamount of glass frit between each of the three members and firing theassembled pressure transducer to fuse the glass frit thereby forming aseal. In the aforementioned embodiment, the conductive layers arescreened onto the discs 2 and 4 using a conductive paste and a smallamount of sealing material applied between the discs 2 and 4 and spacermember 18. The sealing material is preferably a glass frit but may be aceramic based sealing material. The thus assembled pressure transduceris then fired to seal the respective members and complete the assembly.

In operation when pressure is applied to the capacitive transducer 21,one or both discs act as diaphragms and the spacing or gap 7 between thediscs changes. When the two discs deflect under pressure the capacitanceof the transducer changes. Hence the capacitance changes as a functionof pressure and the transducer can be calibrated such that any specificcapacitance equals some specific pressure applied to the transducer. Thechange in capacitance can be measured by many of several different typesof electronic circuits which exist in the art. One of such devices is anA. C. Wheatstone reactance bridge which is well knon in the art.

In FIG. 3 another embodiment of a capacitance pressure transducer isshown in accordance with the teachings of the present invention. Theembodiment of FIG. 3 is similar to that shown in FIGS. 1 and 2 andaccordingly is only shown in cross-section. In FIG. 3 like referencenumerals denote like elements of the embodiment of FIGS. 1 and 2.

In FIG. 3 central conductive layers 6 and 8 of substantially circularshape are applied onto the discs 2 and 4. Also conductive layers 10 and12 are applied respectively onto each of the discs 2 and 4 and form aconductive layer from the central layers 6 and 8 to the outer edge ofdiscs 2 and 4. Similarly, leads 14 and 16 are coupled respectively toconductive layers 10 and 12.

The capacitive pressure transducer 31 is assembled by applying a glassfrit (or ceramic sealing material) 32 in the vicinity of the perimeterof the surface (s) of disc(s) 2 and/or 4 onto which central conductivelayer(s) 6 and/or 8 was previously applied. Disc 2 is then placed on topof the disc 4 with conductive layer 6 opposite conductive layer 8separated by a gap. A discussion of the thickness ranges of the gap andmaterial coating dimensions will be discussed later on in thespecification. The initially assembled capacitive pressure transducer 31is then fired thereby fusing the glass frit 32. When the glass frit 32is fused discs 2 and 4 are bonded and sealed together around theperiphery between them. Furthermore, in this embodiment, a spacer membersuch as the spacer member 18 in the embodiment of FIGS. 1 and 2 is notrequired thereby reducing the number of components required to assemblethe pressure transducer 31.

As in the embodiments of FIGS. 1 and 2, the preferred material for discs2 and 4 is a zero hysteresis nonconductive electrically insulative,elastic material such as alumina, fused silica, or glass such as Pyrex.Furthermore, in the preferred embodiment of the pressure transducer 31,the conductive layers are screened and fired onto the discs 2 and 4using a conductive paste and then a glass frit is applied about theperimeter of disc(s) 2 and/or 4. Disc 2 is then placed on top of disc 4and the thusly assembled pressure transducer is then fired to completethe assembly. Alternatively, the conductive layers 6, 8, 10 and 12 couldbe applied to discs 2 and 4 and glass frit 32 applied to one or bothdiscs. The complete assembly could then be fired to seal the assembly.

In FIG. 4 a third embodiment of a capacitive pressure transducer isshown in accordance with the teachings of the present invention. Theembodiment of FIG. 4 is similar to that shown in FIGS. 1 and 2 andaccordingly is only shown in cross-section. In FIG. 4 like referencenumerals denote like elements to the embodiment of FIGS. 1 and 2.

In FIG. 4 the capactivie pressure transducer 41 includes twononconductive insulative plates (e.g. discs) assembled such that whenplaced together a gap 46 of substantially circular cross-section existsbetween the plates 42 and 44. Circular conductive layers 6 and 8 areapplied onto the inside surfaces of plates 42 and 44. Also conductivelayers 10 and 12 are applied respectively onto each of the plates 42 and44 and form a conductive layer from the central layer 6 and 8 to theouter edge of plates 42 and 44. Leads 14 and 16 are coupled respectivelyto conductive layers 10 and 12.

The capacitive pressure transducer 41 is assembled by applying a smallamount of glass frit about the perimeter of the inner surface of plate44. Plate 42 is then placed on top of plate 44 with conductive layer 6adjacent to an opposite conductive layer 8. The initially assembledcapacitive pressure transducer 41 is then fired thereby fusing the glassfrit. When the glass frit is fused, plates 42 and 44 are bonded togetherby a thin glass seal around the perimeter. Thus, only a very thin glassseal is made between the two materials and not the large seal shown inthe embodiment of FIG. 3. Typically the thickness of the seal is in theorder of one micro inch to one milinch or 10⁻⁶ to 10⁻³ inches.

As in the other embodiments, the preferred material for plates 42 and 44is a zero hysteresis, nonconductive electrically insulative elasticmaterial such as alumina, fused silica or glass such as Pyrex.Furthermore, in the embodiment for pressure transducer 41 the conductivelayers are applied onto the plates 42 and 44 using a conductive pasteand then a small amount of glass frit is applied in the vicinity of theperimeter of plate 44. Plate 42 is then placed on top of plate 44 andthe assembled pressure transducer is then fired to join the two platesand fuse the glass frit to form a seal.

Furthermore, it should be apparent to one skilled in the art that theshape of the plates 42 and 44 can take any number of forms so long asthe gap 46 between the inside surfaces of the plates 42 and 44 isprovided. Typically, the plates 42 and 44 may be formed so that they arethicker on one edge than another, as shown in the drawings (FIG. 4 andFIG. 5) or with one plate thicker than the other to form a stationarysubstrate and diaphragm assembly as shown in FIGS. 8, 9 and 10.Furthermore, the plates could be formed in the shape of a dish, concavesurface, or a shape essentially the same as the delection curve of theplate(s) or disc(s) under pressure.

In FIG. 5 a fourth embodiment of a capacitive pressure transducer isdisclosed in accordance with the teachings of the present invention. Theembodiment of FIG. 5 is similar to that shown in FIGS. 1, 2 and 4 andaccordingly is only shown in cross-section. In FIG. 5 like referencenumerals denote like elements to the other embodiments.

In FIG. 5, the plates 42 and 44, conductive layers 6, 8, 10 and 12 areformed substantially as previously described. Leads 52 and 54 arecoupled to conductive layers 10 and 12.

In order to assemble the capacitive pressure transducer 51, the plates42 and 44 are placed one on top of the other with conductive layers 6and 8 adjacent and opposite each other. The leads 52 and 54 are appliedaround plate 44 such that electrical contact points are available on thebottom surface of plate 44. A glass frit or preform 56 is then appliedto the outside margin of the disc and the combination is then fired tofuse the glass frit. It should be noted that it is within the scope ofthe invention to extend leads 52 and 54 downwardly along the side ofplate 44 and side braze the leads 52 and 54 to conductors 10 and 12.This variant would have application primarily where square orrectangular plates were employed.

In FIG. 6 a fifth embodiment of a pressure transducer is disclosed inaccordance with the teachings of the present invention. The embodimentof FIG. 6 is similar to that shown in FIG. 4 and accordingly is shownonly in cross-section. In FIG. 6 like reference numerals denote likeelements to the other embodiments.

In FIG. 6, the plate 42 is provided with a vent hole 62 for venting theinterior of the capacitive pressure transducer to the outside ambientpressure media. Vent hole 62 is vented to the outside media via a filter64. In this way contaminants are prevented from entering the gap betweenthe plates 6 and 8. In all other ways the capacitive pressure transducer61 is made, assembled and operated in substantially the same way as thecapacitive pressure transducer 41 in FIG. 4. It should be appreciatedthat the capacitive pressure transducer may measure either gaugepressure or absolute pressure depending upon the requirement of theparticular use. Further in practice, in some practical applications theambient media might be air, oil or other media.

In FIG. 7 a sixth embodiment of a capacitive pressure transducer isshown in accordance with the teachings of the present invention. Theembodiment of FIG. 7 is similar to that shown in FIG. 4 and accordinglyis only shown in cross section. In FIG. 7 like reference numerals denotelike elements to the other embodiments.

In FIG. 7 the capacitive pressure transducer 71 includes twononconductive insulative plates formed such that when placed together agap 46 of substantially circular cross-section exists between the plate42 and cylindrical plate 72. Cylindrical plate 72 is formed such that itis substantially thicker than plate 42. Circular conductive layers 6 and8 are applied onto the inside surfaces of plate 42 and cylindrical plate72. Also conductive layers 10 and 12 are applied respectively onto eachof the plates 42 and 72 to form a conductive layer from the centrallayers 6 and 8 to the outer margin of plates 42 and 72. Leads 14 and 16are coupled respectively to conductive layers 10 and 12.

The capacitive pressure transducer 71 is now assembled in a mannersubstantially the same as the capacitive pressure transducer 41 in FIG.4.

In operation, since cylindrical plate 72 is substantially thicker thanplate 42, only plate 42 acts as a diaphragm. In other words, whenpressure is applied to the capacitive pressure transducer 71, in essenceonly plate 42 and accordingly layer 6 is displaced relative to layer 8thereby varying the capacitance.

In FIG. 8 like reference numerals denote like elements to the embodimentof FIG. 7. In FIG. 8 is shown a capacitive pressure transducer 81substantially the same as that shown in FIG. 7 except that thecapacitive pressure transducer 81 is provided with a hybrid circuit 82.

The cylindrical plate 72 is provided on its outside surface with ahybrid circuit 82. Hybrid circuit 82 may be manufactured separately andbonded to cylindrical plate 72 or may be deposited directly ontocylindrical plate 72 utilizing it as a substrate. The hybrid circuit maybe located on any surface having sufficient available area. In someapplication, it may be desirable to form the hybrid ciruit adjacent tothe capacitor plates formed on cylindrical plate 72. Thus, the hybridcircuit would be sealed and protected in applications wherein thecapacitive chamber was not vented.

Another embodiment of the pressure transducer assembly is disclosed inFIG. 9. In FIG. 9 the capacitive pressure transducer 91 includes twoflat surfaced nonconductive insulative disc shaped plates formed suchthat when placed together a gap 96 of substantially circularcross-section exists between the plate 92 and opposing plate 94. Thenonconductive material of which the plates are constructed is preferablyone which has approximately a zero hysteresis, such as alumina, fusedsilica or glass such as Pyrex. In FIG. 9, central conductive layers 6,8, 10 and 12 are formed substantially as previously described. The plate92 is formed so that it is substantially thicker than plate 94, withplate 94 acting as a diaphragm. When pressure is applied to thecapacitive pressure transducer 91, in essence only plate 94 andaccordingly layer 6 is displaced relative to layer 8 thereby varying thecapacitance. Circular conductive layers 6 and 8 are applied to theplanar inside surfaces of the plate 92 and plate 94 as previouslydiscussed. Also conductive layers 10 and 12 are applied respectivelyonto each of the plate surfaces in electrical contact with theconductive layers 6 and 8 to form a conductive layer from the centrallayers 6 and 8 near the outer margin of plates 92 and 94 so that leads14 and 16 can be coupled respectively to conductive layers 10 and 12. Aglass frit 98 which may be either conventional glass frit or a compositeglass frit containing a higher temperature glass which acts as a spaceris applied to one or both of the plates with plate 94 being placed inoverlying position. The assembled capacitive pressure transducer 91 isthen fired fusing the glass frit, spacing and bonding plates 92 and 94together by a thin glass seal around the margins of the plates. Thethickness and ranges of the conductive layers and frit will be discussedin greater detail in the following paragraphs.

The preferred embodiment and best mode of the invention is disclosed inFIGS. 10 through 12 while the complete assembly is shown in FIGS. 13 and14. In these Figures the capacitive pressure transducer 101 comprisestwo nonconductive insulative flat surfaced members spaced and sealedtogether by a glass frit 102 so that the members when sealed togethercreate a predetermined gap 104 of about 0.00155 inches±0.0001 inchesbetween the diaphragm member 106 and the stationary substrate member108. The members can be constructed of fused quartz, or high purityalumina of about 96 % purity. The requirements for the material are thatit has hysteresis free operation, elasticity with necessary tensilestrength, and temperature stability. The preferred material whichpresents an acceptable material interface is high purity alumina. Inthis preferred embodiment the stationary substrate member 108 is severaltimes thicker than the diaphragm member 106. The stationary substratemember as seen in FIG. 11 has a conductive plate 110 comprising acentral conductive plate 112 having a radially extending conductive path113 electrically connected to lead contact 118. A guard ring plate 114substantially surrounds plate 112 with the inner periphery of guard ringplate 114 being separated from the outer periphery of conductive plate112 and its radiating pathway 113 by a nonconductive space 115 having awidth ranging from 0.015 to 0.030 inches. The conductive plate materialcomprises fired gold, screened and fired on the flat member surfae in athickness ranging from 500 to 2500 angstroms. The lead contacts 118,120, 122 and 124 are separately screened and fired on the member flatsurface in a somewhat thicker range. The reason for the double screeningis that pure gold material of the lead contact can be leached onto thefrit glass or plate material making it nonconductive. It should also benoted that platinum can be substituted for gold as the electricallyconductive medium, and because of its superior leach resistance only asingle screening can provide 112, 113, 118, 120, 122 and 124.

A glass frit 102 having a thickness of about 0.0015 inches is screenedand dried on the outer margin of the stationary substrate plate in anannular configuration. The frit is applied so that it does not coverlead holes 119 and 121 defined by contacts 118 and 120 respectively bymore than half their diameter, allowing respective lead wires 126 and128 to be inserted therein. The applied frit also leaves a portion ofthe ground lead contacts 122 and 124 exposed. The central conductiveplate 112 is screened onto the planar surface of the stationarysubstrate member with a series of nonconductive semi-circular spaces116. Ground contacts 122 and 124 provide leads to the outer guard ring114. An opening 123 is formed exterior of the frit in contact 124 forinserting an external test lead. An evacuation bore 140 as better shownin FIG. 10 leads into the capacitor chamber through the guard ring 114and is sealed over by a glass sealing bead 142 so that the chamber canbe placed in a vacuum, or with one or more atmospheres of pressuredepending upon the use desired.

The diaphragm member 106 is composed of the same material as thesubstrate member 108 with the conductor plate 130 being constructed byscreening fired gold onto the flat planar surface in the thicknessespreviously described.

Glass frit 102 is screened onto the margin of the planar surface of thediaphragm member 106 around the periphery of the guard ring in the samemanner as was the case in the substrate member. However, it is apparentvarious frit thicknesses can be used for either plate as long as the gapbetween the plates created by the fused frit preferably ranges between0.00145 inches and 0.00165 inches. However the gap between the plateshas been found to be workable between 0.001 and 0.002 inches. Theconductive plate 130 screened onto the flat surface of the diaphragmmember in the thickness and manner previously discussed regarding thesubstrate member so that it is separated in a central conductive plate132 with an outward radiating path 133 and an outer guard ring 134 by anonconductive space 131 having a width ranging from 0.015 to 0.030inches. It should be noted that point X of the diaphragm member is to bepositioned directly opposite and facing point Y of the substrate memberin assembly of the member to form the capacitive transducer. Groundcontacts 222 and 224 are disclosed leading to the guard ring 34. Thesecontacts function in the same manner as contacts 122 and 124. Contact218 is electrically connected to the central conductive plate 132 bypathway 133. It should be noted that contacts 218, 222 and 224 formedbetween the diagraphm member surface and the frit 102 provide electricalconnection through the frit to the central conductive plate 132 andguard ring 134 respectively.

The complete pressure transducer sensor assembly 150 is shown in FIG.14. The pressure assembly system comprises pressure transducer 101previously described in the preceding paragraphs, and a laser trimmedelectronic circuit 152 which is secured to an appropriate side of thesensor 101 which in the preferred embodiment is the stationary substrateside. It should be noted that the electronic circuit has leads 154 and156 which can be connected to leads 126 and 128 of sensor 101 or whichmay be substituted for those leads. An upper sealing gasket 158 isplaced over the electronic circuit 152 and substrate plate and a lowersealing gasket 160 is placed over the diaphragm section. The gaskets areheld in place by an electrical shield and cover 162 which is mounted tothe upper sealing gasket 158 at 164. An electrical shield and bezel 166is mounted over the lower sealing gasket 160 so that rim 168 of thelower sealing gasket protrudes therethrough with a flange 170 of theelectrical shield 166 resting on an annular seat 172 of the lowersealing gasket. Two of the transducer assemblies 150 are then mountedinto a housing not shown which connects them to the vent and manifold ofan internal combustion engine. The rear of the hybrid electronics member152 is connected to a printed circuit board assembly to control theelectronic ignition of the internal combustion engine.

In the above-described embodiments of the capacitive pressure transducerand particularly the preferred embodiment, gaps between the plates onthe order of 0.00145 to 0.00165 inches are practical. Typically, thedeflecting portion of the plates of the diaphragm when made from aluminahas a thickness ranging from approximately 0.001 to 0.500 inches.Furthermore, the thickness of the plates and the width of the gap can bedesigned such that under high overload pressures the two discs bottomout against each other thereby preventing damage to the pressure sensor.Also it should be apparent to one skilled in the art that the physicalshape of the plates is no determinative of the invention and that thepressure transudcer could be just as easily made from square orrectangular plates of a nonconductive or insulating material.Furthermore, the central conductive layer need not be circular and alsocould be square, rectangular or any other shape required.

In all cases it is understood that the above-described embodiments aremerely illustrative of a number of the many possible specificembodiments which can represent applications of the principles of thepresent invention. Numerous and various other arrangements can bereadily devised in accordance with these principles by those skilled inthe art without departing from the scope of this invention.

What is claimed is:
 1. A method for making a capacitive pressuretransducer comprising:preparing two members of substantially zerohysteresis nonconducting material with flat matching opposed surfaces,at least one of said members being thin enough to be flexible; applyingan electrically conductive layer onto each of two nonconductive membersin a predetermined pattern, comprising a central portion defining anonconductive cut out portion and an outer portion substantiallysurrounding said central portion and spaced away from said centralportion; firing the two nonconductive members; applying glass frit aboutthe marginal perimeter of at least one of said members; disposing onemember onto the other with the fired conductive layers facing eachother, with the glass frit forming the only spacing between the flatmatching surfaces of said members; and firing the members therebybonding the two members together such that said conductive layers areopposite each other, and separated by a gap produced only by said glassfrit, insulated one from the other, and sealed around the periphery ofsaid members by said glass frit.
 2. A method as defined in claim 1including the additional steps of mounting an integrated circuit ontoone of said members and coupling said opposed conductive plates to saidintegrated circuit.
 3. An economical, high production method for makingprecision capacitive pressure transducers comprising the stepsof:preparing two members of substantially zero hysteresis nonconductingmaterial with flat matching opposed surfaces, and the opposing surfacesof each member lying substantially in a single plane, and at least oneof said members being thin enough to be flexible; screening anelectrically conductive layer onto said flat surface of each of said twononconductive members in a predetermined pattern for each member, with amain central area and outwardly extending conductive pathways forexternal electrical connections; firing the two nonconductive members tobond the conductive layers to the nonconductive members; applying a thinlayer of glass frit around the main central conductive area onto saidflat surface of at least one of said nonconducting members with saidoutwardly extending conductive pathways extending beyond said frit;disposing one member onto the other with the fired conductive layersfacing each other, and with the glass frit forming the only spacingbetween the planar opposing surfaces of said members; firing themembers, thereby bonding the two members together such that saidconductive layers are opposite each other, and separated by a gap lessthan 0.010 inch in thickness produced only by said glass frit engagingboth of said planar opposed surfaces over an outwardly extendingdistance substantially greater than the thickness of said gap, insulatedone from the other, and substantially sealed around the main conductiveareas on said members by said glass frit with said outwardly extendingconductive pathways extending through the seal provided by the firedglass frit; and making electrical connections to said outwardlyextending pathways, outside of said glass frit to permit detection ofchanges in the capacitance between said conductive layers as thepressure applied to said transducer varies.
 4. A method as defined inclaim 3 including the step of making the outwardly extending conductivepathways thicker than the central conductive area to insure theretention of their conductivity despite contact with the glass fritduring firing.
 5. A method as defined in claim 3 wherein said glass fritincludes a high and a lower melting point glass powder and wherein thefiring of said glass frit is performed at a temperature between said twomelting points, whereby the spacing of said plates is determined bythickness of the non-melting frit.
 6. An economical, high productionmethod for making precision capacitive pressure transducers comprisingthe steps of:preparing two plates of substantially zero hysteresisnonconducting material with flat matching opposed surfaces, and theopposing surfaces of each member lying substantially in a single plane,at least one of said plates being thin enough to be flexible; screeningan electrically conductive layer onto each of said two plates in apredetermined pattern for each member, with a main central area andoutwardly extending conductive pathways for external electricalconnections; firing the two plates to bond the conductive layers to theplates; applying a thin layer of glass frit around the main centralconductive area on at least one of said nonconducting plates with saidoutwardly extending conductive pathways extending beyond said frit;disposing one member onto the other spaced apart by a distance of aboutone-half to several thousandths of an inch, with the fired conductivelayers facing each other, and with the glass frit extending between theplanar opposing surfaces of said members; firing the members, therebybonding the two members together such that said conductive layers areopposite each other, and separated by a gap less than 0.010 inch inthickness; insulated one from the other, and substantially sealed aroundthe main conductive areas on said members by said glass frit engagingboth of said planar opposed surfaces over an outwardly extendingdistance substantially greater than the thickness of said gap, with saidoutwardly extending conductive pathways extending through the sealprovided by the fired glass frit; and making electrical connections tosaid outwardly extending pathways, outside of said glass frit, to permitdetection of changes in the capacitance between said conductive layers,as the pressure applied to said transduer varies.
 7. An economical, highproduction method for making precision capacitive pressure transducerscomprising the steps of:preparing two members of substantially zerohysteresis nonconducting material with flat matching opposed surfaces,and the opposing surfaces of each member lying substantially in a singleplane, at least one of said members being thin enough to be flexible;applying electrically conductive layer onto each of said twononconductive members in a predetermined pattern for each member, with amain central area and outwardly extending conductive pathways forexternal electrical connections; applying a thin layer of ceramicsealing material around the main central conductive area on at least oneof said nonconducting members with said outwardly extending conductivepathways extending beyond said ceramic sealing material, said ceramicsealing material having a coefficient of thermal expansion substantiallyequal to that of said nonconductive members, disposing one member ontothe other with the conductive layers facing each other, and with theceramic sealing material extending between the planar opposing surfacesof said members; firing the members, thereby bonding the two memberstogether such that said conductive layers are opposite each other,separated by a gap less than 0.010 in thickness produced only by saidceramic sealing material, insulated one from the other, and sealedaround the main conductive areas on said members by said ceramic sealingmaterial engaging both of said planar opposed surfaces over an outwardlyextending distance substantially greater than the thickness of said gap,with said outwardly extending conductive pathways extending through theseal provided by the fired ceramic sealing material; and makingelectrical connections to said outwardly extending pathways, outside ofsaid ceramic sealing material to permit detection of changes in thecapacitance between said conductive layers, as the pressure applied tosaid transducer varies.
 8. An economical, high production method formaking precision capacitive pressure transducers comprising the stepsof:preparing two members of substantially zero hysteresis nonconductingmaterial with flat matching opposed surfaces, and the opposing surfacesof each member lying substantially in a single plane, and at least oneof said members being thin enough to be flexible; screening anelectrically conductive layer onto said flat surface of each of said twononconductive members in a predetermined pattern for each member, with amain central area for forming a capacitor plate; firing the twononconductive members to bond the conductive layers to the nonconductivemembers; applying a thin layer of glass frit around the main centralconductive area onto said flat surface of at least one of saidnonconducting members; disposing one member onto the other with thefired conductive layers facing each other, and with the glass fritextending between the planar opposing surfaces of said members; firingthe members, thereby bonding the two members together, such that saidconductive layers are opposite each other, separated by a gap less than0.010 inch in thickness, insulated one from the other, and sealed aroundthe main conductive areas on said members by said glass frit engagingboth of said planar opposed surfaces over an outwardly extendingdistance substantially greater than the thickness of said gap; andmaking electrical connections to each of said conductive layers, topermit detection of changes in the capacitance between said conductivelayers as the pressure applied to said transducer varies.
 9. A method asset forth in claim 8 including the step of applying glass frit which hasa temperature coefficient of expansion which is substantially equal tothat of said nonconducting members, thereby minimizing mechanicalstresses during subsequent heating and cooling steps.
 10. A method asdefined in claim 9 wherein said glass frit is screened onto theperiphery of at least one of the opposed surfaces of one of saidmembers.
 11. A method as defined in claim 9 wherein two coats of fritare applied to surfaces of said members at the periphery thereof.
 12. Amethod as defined in claim 8 including the step of preparing said twononconductive members as thin flexible diaphragms with substantiallyparallel top and bottom surfaces.
 13. A method as defined in claim 8including the step of locating elements of material between said flatopposed surfaces of said nonconducting members, said elements ofmaterial having a melting point above that of at least a portion of saidfrit, to space said members apart by a predetermined distance.
 14. Amethod as defined in claim 13 wherein said elements of material are highmelting point particles of glass powder.
 15. A method for making acapacitive pressure transducer as claimed in claim 8 where the step offiring the members thereby bonding the two members together is followedby an additional step of securing a laser-trimmed integrated circuit tothe outer surface of one of said nonconductive members.
 16. A method asdefined in claim 8 including the additional steps of depositing anintegrated circuit directly onto one of said members and coupling saidopposed conductive plates to said circuit.
 17. A method for making acapacitive pressure transducer comprising:preparing two members ofsubstantially zero hysteresis nonconducting material with planarmatching opposed surfaces, at least one of said members being thinenough to be flexible; applying an electrically conductive layer ontoeach of the two nonconductive members in a predetermined pattern to formthe plates of the capacitive transducer; applying glass frit about themarginal perimeter of at least one of said members; disposing one memberonto the other with the conductive layers facing each other with theglass frit between the flat matching surfaces of said members and themembers and their respective associated conductive layers spaced apredetermined distance apart; and firing the members thereby bonding thetwo members together such that said conductive layers are opposite eachother, and separated by a gap having said predetermined distance lessthan 0.010 inch, insulated by a gap having said predetermined distance,insulated one from the other, and sealed around the periphery of saidmembers by said glass frit engaging both of said planar opposed surfacesover an outwardly extending distance substantially greater than thethickness of said gap.
 18. A method for making a capacitive pressuretransducer comprising:preparing two members of substantially zerohysteresis nonconducting material with flat matching opposed surfaces,at least one of said members being thin enough to be flexible; screeningan electrically conductive layer onto each of two nonconductive membersin a predetermined pattern, comprising a central portion defining anonconductive cut-out portion and an outer portion substantiallysurrounding said central portion and spaced away from said centralportion; applying glass frit having a coefficient of expansionsubstantially the same as the two members about the marginal perimeterof at least one of at least one of said members; disposing one memberonto the other with the conductive layers facing each other, with theglass frit forming the only spacing between the flat matching surfacesof said members to form a predetermined spacing; and firing the membersthereby bonding the two members together such that said conductivelayers are opposite each other, and separated by a gap produced only bysaid glass frit, insulated one from the other, and sealed around theperiphery of said members by said glass frit.
 19. A method forfabricating a capacitive pressure sensor comprising the steps of:providing two glass plates, at least one of said glass plates beingflexible enough to be capable of responding to a change in pressure, andeach having a substantially plane surface; providing conductiveelectrodes on said substantially plane surfaces; applying a sealingglass material to at least one of said two glass plates; positioningsaid two glass plates to place said conductive electrodes in asubstantially parallel relationship having a space therebetween;evacuating the space between said spaced apart electrodes; heating saidglass plates to cause said sealing glass material to flow and to sealsaid glass plates together, said glass plates and said sealing glassmaterial bounding an evacuated volume.
 20. A capacitive sensor formeasuring ambient pressure comprising:two glass pieces, at least one ofsaid glass pieces being capable of flexing in response to a change inpressure, each having a substantially plane surface with conductiveelectrodes thereon, said glass pieces being positioned to place saidconductive electrodes in a spaced apart, substantially parallelrelationship; a sealing glass mixture disposed on one of said pieces toseal together said glass pieces, said mixture and said glass piecesacting together to bound a volume having a reference pressure thereinwhereby changes in ambient pressure cause a change in the capacitancemeasured between said conductive electrodes.
 21. The capacitive sensorof claim 20 wherein said two glass pieces comprise a substantially rigidbase piece and a thinner, flexible diaphragm piece.
 22. The capacitivesensor of claim 20 wherein said reference pressure is a substantialvacuum.